This invention relates to an impressed current anode bed which provides for anchoring anodes on the bottom of a body of water. The anode bed is configured to resist movement from the original anchored position and to resist buildup of solids, e.g., silt, around the anodes.
Two common methods for providing cathodic protection for minimizing corrosion of metals submerged in a liquid electrolyte, e.g., sea water, fresh water, etc., are the sacrificial-anode method and the impressed current method. In both methods, an electrical current is generated to make the submerged metal act as the cathode in the system. The sacrificial-anode method places the anode in the electrolyte and generates the required electrical current by corrosion of the anode material. This method is not always the method of choice as replacement of the sacrificed anodes can be very expensive. This is especially true in those cases where the metal to be protected is a portion of an offshore oil platform, a wharf or a bridge which is located in a deep or fast-moving body of water. In these cases, the impressed current method is usually more attractive.
In the impressed current method, the electrical current is provided by an external DC current source. This DC current is provided to an essentially non-sacrificial anode which is submerged in the electrolyte and located in proximity to the metal to be protected. The prior art teaches that the anode can be: of the composite type, e.g., an LIDA anode, manufactured by Oronzio De Nora, S.A., Lugano Switzerland; of an iron alloy, e.g., an iron-silicon-chrome alloy; of a graphite composition; or of a precious metal such as platinum. The composite type and the graphite composition type are provided in lightweight tubular form while the iron alloy is generally provided as a heavy rod. Most often a plurality of anodes are used together in a group and, in any particular group, they are electrically connected together.
Submerging and locating the anode(s) in proximity to the metal to be protected is conveniently achieved by providing an anode bed which has an overall specific gravity greater than that of the electrolyte. An inexpensive anode bed, and one which is widely used, comprises two long logs and the anodes which are all assembled to form a ladder-like structure with the logs being the legs and the anodes being the ladder rungs. If the anodes are heavy, e.g., the iron alloy type, they will supply the submerging ballast. If, on the other hand, lightweight anodes are used, e.g., the composite type, then concrete columns can be substituted for the logs which will assure submergence of the bed. The electrical connection between the anodes is by way of anode end seals and connecting wires, all of which are exposed to the electrolyte. This type of bed is generally satisfactory for use in those bodies of water were high solids concentrations and fast currents, at the bed location, are not a problem. High solids concentrations are troublesome as this bed configuration sufficiently slows the current so that solids carried by the current will fall out and buildup about the bed. This solid buildup is particularly detrimental when the anode surfaces become covered so that they lose their capacity to generate the needed electrical current. The presence of fast water currents, i.e., currents of 10 to 20 mph (16 km/hr to 32 km/hr), can be troublesome as these currents can move solids, e.g., silt, sand, etc., into erosive contact with the exposed anode end caps and connecting wires which results, over time, in their failure.
Therefore, it is an object of this invention to provide an impressed current anode bed which is highly resistant to an accumulation of solids which would otherwise cover the anode surfaces. It is also an object of this invention to provide such a bed which protects against erosion of the electrical connections between the anodes.